U.S. Pat. No. 5,905,407 discloses a high efficiency power amplifier using combined linear and switching techniques with a feedback system. A linear amplifier supplies an output current to a load via a sense resistor. A switching amplifier which comprises a controllable switch and two LC-sections is used as a DC-DC converter and supplies a further output current to the load. The resistor is arranged between the output of the linear amplifier and the output node of the power supply system at which the output voltage is present across the load. The output current of the linear amplifier flows through this resistor. The voltage across the resistor is used to control the DC-DC converter to obtain a minimal DC-component of the output current of the linear amplifier. Preferably, this minimal DC component is zero.
This parallel arrangement of a linear amplifier and a DC-DC converter is applied in a radio transmitter. The parallel arrangement is further also referred to as a power supply system. The radio transmitter comprises a power supply reference generator which supplies a reference signal to the linear amplifier to generate the system output voltage which tracks the reference signal. The radio transmitter further comprises a radio frequency (further referred to as RF) power amplifier for amplifying an RF signal. The RF amplifier is coupled to the output node to receive the system output voltage as a supply voltage. The reference signal is modulated to follow an amplitude modulation of the input signal of the RF amplifier. Thus, the supply voltage of the RF amplifier is controlled to meet the needs of the RF power amplifier to improve the efficiency of the RF amplifier.
The relatively slow DC-DC converter supplies the DC and low frequent currents to the load at a relatively high power efficiency, and the relatively low power efficiency linear amplifier supplies only the high frequent currents to the load.
The prior art resistor arranged between the linear amplifier and the output node has the drawback that it decreases the efficiency of the power supply system.